General synonyms:
* Firm
* Steadfast
* Resolute
* Determined
* Inflexible
* Impregnable
* Unyielding
* Rigid
* Stubborn
* Intransigent
More specific synonyms:
* Relentless (for persistent effort)
* Indomitable (for unbreakable spirit)
* Impervious (for resistance to influence)
* Unbending (for inflexible adherence to principles)
* Iron-willed (for unwavering determination)
The best synonym to use depends on the specific meaning you want to convey.